group by grouping sets 的用法

SELECT country, null department, round(avg(salary), 2) FROM employee1 GROUP BY country
UNION ALL
SELECT null country, department, round(avg(salary), 2) FROM employee1 GROUP BY department;
等价于
SELECT country, department, round(avg(salary), 2) FROM employee1 GROUP BY GROUPING SETS (country, department);


GROUP BY GROUPING SETS (A,B,C)  等价与  GROUP BY A  
                                        UNION ALL  
                                        GROUP BY B  
                                        UNION ALL  
                                        GROUP BY C
                                        
                                        
GROUP BY GROUPING SETS ((A,B,C))  等价与  GROUP BY A,B,C  
   
   
GROUP BY GROUPING SETS (A,(B,C))  等价与  GROUP BY A  
                                          UNION ALL  
                                          GROUP BY B,C

                                        
GROUP BY GROUPING SETS (A)  等价于  GROUP BY A,B,C  
        ,GROUPING SETS (B)  
        ,GROUPING SETS (C)  
   
   
GROUP BY GROUPING SETS (A)  等价于  GROUP BY A,B,C  
        ,GROUPING SETS ((B,C))   
   
   
GROUP BY GROUPING SETS (A)  等价于  GROUP BY A,B  
        ,GROUPING SETS (B,C)        UNION ALL  
                                    GROUP BY A,C
                                    
                                    
GROUP BY A                     等价于  GROUP BY A  
        ,B                                     ,B  
        ,GROUPING SETS ((B,C))                 ,C  
   
   
GROUP BY A                    等价于  GROUP BY A,B,C  
        ,B                            UNION ALL  
        ,GROUPING SETS (B,C)          GROUP BY A,B  
   
   
GROUP BY A                    等价于 GROUP BY A,B,C  
        ,B                           UNION ALL  
        ,C                           GROUP BY A,B,C  
        ,GROUPING SETS (B,C)